Companies need networks to communicate both internally and externally. For internal communications, some companies create infrastructure, such as optical cables for transmitting electronic communications. Such infrastructure may also be leased on an exclusive basis from telecom companies, or may be shared with other companies. Still further, telecom companies can provide a virtual private network (VPN), essentially transmitting packets of voice and data via public infrastructure. A VPN may be thought of as a private communications network usually used within a company, or by several different companies or organizations, to communicate over a public network. VPN message traffic is carried on public networking infrastructure (e.g. the Internet) using standard (often insecure) protocols, or over a service provider's network providing VPN service guarded by well defined Service Level Agreement (SLA) between the VPN customer and the VPN service provider.
Companies can interface to the public network via provider edge (PE) line cards and customer edge (CE) line cards. A PE line card is part of a router between one network service provider's area and areas administered by other network providers. A CE line card is part of a router that is owned by a customer and provides routing of traffic within a customer.
Many companies may interface to a PE router via CE routers. The PE has a customer side, and a core facing side, where the core is the public network. Core facing line cards each have an internet routing table that currently consists of about 150,000 routes. This consumes a significant amount of memory, and increases the cost of such line cards. Each customer may also have their own private routing tables. Such routing tables may have about 5,000 routes apiece. Including all the private routing tables, as well as the internet routing table would consume even more resources of line cards, and hence significantly increase the cost of such line cards, which are already expensive.